they are not versatile and universal (i.e. a kit to build one particular hexapod or line-follower, not a kit for do it anything). Are all universal kit systems (meccano, lego) patented?
And legos, more than most things, are thus programmable structures. Recall that lots of early computers (for example, the one on Apollo missions) were NOT re-programable. The code was encoded in the physical form, and couldn't be moved around without essentially re-manufacturing the device. The step to programable computers was a major milestone.
Legos make a similar leap in an engineering space instead of a computational space; it's super cool.